Bernie Sander showed his support for Hillary Clinton after losing out in 2016

"Together, you and I and our 2016 campaign began the political revolution. Now, it is time to complete that revolution and implement the vision that we fought for," Sanders will say in an email to supporters announcing his candidacy.

The 77-year-old Vermont independent senator, who started his political career as a gadfly perennial candidate, remains a pacesetter of progressive politics in America.

Sanders pushes a liberal socialist agenda that includes everything from Medicare for All to a $15 minimum wage to free college tuition.

And early polls show him far ahead of the rest of the pack and trailing only former Vice President Joe Biden in the nascent 2020 field.

"Three years ago, during our 2016 campaign, when we brought forth our progressive agenda we were told that our ideas were 'radical,' and 'extreme,'" Sanders said in the email.

"These policies and more are now supported by a majority of Americans."

Many Clinton supporters blame Bernie Sanders for her losing to President Trump

But with greater expectations comes less room for error and, given the size of the still-growing 2020 field, potentially less room to grow.

Sanders will come under more scrutiny than ever before from both the press and political rivals, including from sexual harassment allegations against 2016 campaign staffers that have roiled his campaign-in-waiting in recent months.

And instead of being the main alternative to a prohibitive frontrunner, as he was against Hillary Clinton, Sanders now faces stiff competition from a wide array of candidates for his core supporters of progressives and young people.

Even some of Sanders' former staffers have already signed on with other candidates, though many have remained loyal.

And some Democrats remain bitter about 2016, accusing Sanders and his followers of damaging Clinton in ways that contributed to her defeat against Donald Trump.

But at the same time, Sanders will be much less lonely in the Democratic Party of 2019 than the one that existed in 2015 when he entered the presidential race.